This application is a continuation of 10/394,585 filed Mar. 24, 2003 now U.S. Pat. No. 6,983,555.
1. Field of the Invention
The present invention relates to footwear, and in particular to an article of footwear designed to accommodate vertical forces and horizontal shear forces, both acting as the result of a foot strike, change in motion of the wearer, or both.
2. Background of the Invention
Soles in footwear, and especially athletic footwear, are designed to provide cushioning and stability. The cushioning aspect is normally designed to minimize the impact in the vertical direction caused when the wearer's body weight, moving in a downward vertical direction, acts on a wearer's foot as it strikes the ground. The stability feature is necessary to control the amount of horizontal motion of a wearer's foot in relation to a securely planted outsole of the footwear.
Historically, due to a focus on the negative effects of vertical forces resulting from footstrikes during walking and running, many attempts have been made at providing optimal vertical shock absorption.
During normal walking or running, the largest forces acting on a wearer's body are in the vertical direction. However, horizontal shear forces are also acting on a wearer's body. For example, as the foot of a person strikes the ground, the heel strikes first. The foot then rolls forwardly and inwardly over the ball of the foot. During the time that the foot is rolling forward, the foot also pronates, a process by which the foot rolls from the lateral side to the medial side. This pronation causes horizontal shear forces to act on the wearer's foot. The lateral motion of the foot resulting from the horizontal shear forces can be controlled by providing stability in the sole of the footwear. However, as the horizontal stability of the footwear increases, the horizontal shock absorption properties of the footwear decrease.
Horizontal shear forces also act on a wearer's body during starting, stopping, and shifting of direction, due to friction between the ground and the shoe. This force of friction is transferred by the shoe to the wearer's foot. Such horizontal shear forces may cause injury to the wearer's ankles if the friction causes the shoe to stop before the wearer's foot can adjust to the change of motion. Attempts have been made to reduce the impact of horizontal shear forces on a wearer's body. For example, posting in a shoe helps to prevent over-pronation of the foot. Once again however, as the stability of such footwear has been increased to accommodate for the horizontal shear forces, the horizontal and vertical shock absorption properties of the footwear have decreased.
Accordingly, a need exists to develop footwear that provides optimal horizontal stability with optimal horizontal absorption properties.